1. Field of the Invention
The present invention relates to a method for reducing an .alpha.,.beta.-unsaturated ketone, and more particularly to a method for obtaining a deoxy-compound by selectively reducing the carbonyl group of the .alpha.,.beta.-unsaturated ketone.
2. Description of the Related Art
Deoxygenation involving the reduction of an .alpha.,.beta.-unsaturated ketone has been used for the synthesis of various medicines or medical materials. For example, it has been tried to obtain a deoxy-compound represented by the following Formula V, which is a useful starting material for preparing prostaglandins and derivatives thereof, by subjecting an .alpha.,.beta.-unsaturated ketone represented by the following Formula III to a 1,2-reduction. When the 1,2-reduction is carried out according to conventional methods, the deoxy-compound V cannot be obtained by a single step but by complicated multiple steps as described below. ##STR1## In the above reaction scheme, Pr is a hydroxy-protecting group.
Conventional methods for 1,2-reduction of the .alpha.,.beta.-unsaturated ketone III are always accompanied by a 1,4-reduction as a side reaction. In the 1,4-reduction, the carbonyl group of the .alpha.,.beta.-unsaturated ketone III is reduced to a hydroxyl group, and a double bond is migrated. As a result, a by-product VII which does not have a double bond is formed from the .alpha.,.beta.-unsaturated ketone III via an intermediate VI.
Conventional methods for obtaining the deoxy-compound V by subjecting the .alpha.,.beta.-unsaturated ketone III to the 1,2-reduction are known, in which a carbonyl group is reduced by two steps. According to one exemplary method, the carbonyl group of the .alpha.,.beta.-unsaturated ketone III is reduced to obtain an allyl alcohol compound IV, and then the allyl alcohol compound IV is dehydroxylated to the deoxy-compound V. Another exemplary method is described in Journal of Organic Chemistry, Vol. 43, p. 2299 (1978). According to this method, the .alpha.,.beta.-unsaturated ketone III is converted to the corresponding hydrazone, and thereafter, the hydrazone is reduced to obtain the deoxy-compound V. These methods have the following problems:
In the former method, when the .alpha.,.beta.-unsaturated ketone III is reduced, the 1,4-reduction is simultaneously effected as described above. This causes the generation of the above-mentioned by-product VII, etc. together with the allyl alcohol compound IV. For this reason, a complicated step for separating the allyl alcohol compound IV from the by-product VII, etc. is required, resulting in poor productivity. In the latter method, the reduction of hydrazone causes the migration of the double bond. Thus, it is not possible to reduce only a carbonyl group.
Alternatively, a method for reducing a carbonyl group has been reported in Synthesis, p. 639 (1974), in which the .alpha.,.beta.-unsaturated ketone is allowed to react with a silyl compound, using trifluoroacetic acid as an acid catalyst. According to this method, a great amount of by-product is generated by the 1,4-reduction; therefore, a deoxy-compound generated by the 1,2-reduction cannot be selectively obtained.